fix(device-profiler): display error message instead of [object Object] (#165)

Add formatError() helper to properly serialize error objects for display.
Handles null, undefined, strings, Error instances, objects with message
property, and JSON-serializable objects. Includes try/catch fallback for
circular references and BigInt values.

Closes #164

Author: groupsky

packages/device-profiler/src/console-formatter.test.ts

@@ -203,4 +203,93 @@ describe('formatSummary', () => {
 
     expect(output).toContain('12.3')
   })
+
+  it('should handle object errors gracefully', () => {
+    // Bug scenario: error field contains an object instead of string,
+    // which was displayed as "[object Object]" instead of a readable message
+    const results: ScanResult[] = [
+      {
+        address: 78,
+        type: RegisterType.Holding,
+        success: false,
+        timing: 3202.3,
+        error: { code: 'EBUSY', message: 'Port is busy' } as unknown as string,
+        errorType: ErrorType.Unknown,
+      },
+    ]
+
+    const output = formatSummary(results)
+
+    // Should not display [object Object]
+    expect(output).not.toContain('[object Object]')
+    // Should display stringified error
+    expect(output).toContain('Port is busy')
+  })
+
+  it('should handle null error', () => {
+    const results: ScanResult[] = [
+      {
+        address: 0,
+        type: RegisterType.Holding,
+        success: false,
+        timing: 100,
+        error: null as unknown as string,
+        errorType: ErrorType.Unknown,
+      },
+    ]
+
+    const output = formatSummary(results)
+    // Count occurrences of '-' (should have at least 2: value and error)
+    const dashes = (output.match(/-/g) ?? []).length
+    expect(dashes).toBeGreaterThanOrEqual(2)
+  })
+
+  it('should handle undefined error', () => {
+    const results: ScanResult[] = [
+      {
+        address: 0,
+        type: RegisterType.Holding,
+        success: false,
+        timing: 100,
+        errorType: ErrorType.Unknown,
+      },
+    ]
+
+    const output = formatSummary(results)
+    const dashes = (output.match(/-/g) ?? []).length
+    expect(dashes).toBeGreaterThanOrEqual(2)
+  })
+
+  it('should handle Error instance', () => {
+    const results: ScanResult[] = [
+      {
+        address: 0,
+        type: RegisterType.Holding,
+        success: false,
+        timing: 100,
+        error: new Error('Connection refused') as unknown as string,
+        errorType: ErrorType.Unknown,
+      },
+    ]
+
+    const output = formatSummary(results)
+    expect(output).toContain('Connection refused')
+  })
+
+  it('should handle object without message property', () => {
+    const results: ScanResult[] = [
+      {
+        address: 0,
+        type: RegisterType.Holding,
+        success: false,
+        timing: 100,
+        error: { code: 'EBUSY', errno: -16 } as unknown as string,
+        errorType: ErrorType.Unknown,
+      },
+    ]
+
+    const output = formatSummary(results)
+    // Should JSON stringify the object
+    expect(output).toContain('EBUSY')
+  })
 })

packages/device-profiler/src/console-formatter.ts

@@ -6,6 +6,19 @@ import Table from 'cli-table3'
 
 import type { ScanResult } from './register-scanner.js'
 
+function formatError(error: unknown): string {
+  if (error == null) return '-'
+  if (typeof error === 'string') return error
+  if (error instanceof Error) return error.message
+  if (typeof error === 'object' && 'message' in error) return String(error.message)
+  try {
+    return JSON.stringify(error)
+  } catch {
+    // Fallback for circular references or BigInt - return type name
+    return `[${typeof error}]`
+  }
+}
+
 /**
  * Format progress message
  *
@@ -34,7 +47,7 @@ export function formatSummary(results: ScanResult[]): string {
     const value = result.success && result.value ? result.value.toString('hex').toUpperCase() : '-'
 
     const status = result.success ? 'OK' : 'FAIL'
-    const error = result.error ?? '-'
+    const error = formatError(result.error)
     const timing = result.timing.toFixed(1)
 
     table.push([result.address.toString(), result.type, status, value, timing, error])